In hydrostatic axial piston machines of this kind, a revolving cylinder drum is provided, in which a plurality of cylinder bores, in each of which a working piston is guided, is arranged in a manner distributed around the circumference. In this arrangement, the cylinder bores and the working pistons extend approximately parallel to a drive shaft of the axial piston machine. Since the piston feet are coupled to a stationary surface, which is set obliquely to the working pistons, a force in the circumferential direction of the cylinder drum is obtained when the working pistons are subjected to pressure (depending on their current pressurization and rotational position on the circumference of the cylinder drum). In operation as a motor, an output torque can thus be taken off at the drive shaft. In operation as a pump, an input torque must be supplied.
In order to the displacement of the hydrostatic axial piston machine, the oblique surface is formed on a swashplate, the pivoting angle of which can be adjusted relative to the longitudinal direction of the working pistons and relative to the axis of rotation of the drive shaft. Because of the pivotability, the swashplate is then also referred to as a pivoting cradle. The pivoting of the pivoting cradle is generally accomplished by means of an adjusting piston, which can engage on the pivoting cradle at a point on the edge of the pivoting cradle remote from the central axis of the drive shaft and from the pivoting axis of the pivoting cradle. To pivot the pivoting cradle back, pressure medium is released from an adjusting chamber on the adjusting piston, enabling a smaller counter piston, which is coupled to the pivoting cradle at an opposite edge section, to act, for example.
The disadvantage with hydrostatic axial piston machines of this kind is the expenditure on equipment for the return motion of the pivoting cradle.
The publication U.S. Pat. No. 4,581,980 shows a hydrostatic axial piston machine having a pivoting cradle, the pivoting angle of which can be adjusted by means of an adjusting piston. The adjusting piston can produce a pivoting moment about the pivoting axis of the pivoting cradle, said pivoting moment acting in the direction of a reduction in the pivoting angle. Opposing this there is a continuous counter pivoting moment or a return pivoting moment applied to the pivoting cradle, said moment being smaller than the pivoting moment that can be applied by the adjusting piston. The counter pivoting moment is produced by arranging the pivoting axis eccentrically with respect to the pivoting cradle, not centrally. The pivoting axis is at a distance from the axis of rotation of the drive shaft and extends in a plane perpendicular to the axis of rotation of the drive shaft. Two circumferential sections of the swashplate of unequal size are defined by the pivoting axis, and therefore there are always more working pistons supported (via the piston feet thereof) on the larger circumferential section of the pivoting cradle than on the remaining, smaller circumferential section. There is therefore a disequilibrium in the sum of the supporting forces, and this acts as an internal counter pivoting moment. Particularly in the case of embodiment as a motor, it is possible to dispense with an additional counter pivoting device in such an axial piston machine since a pressure and hence a counter pivoting moment are built up at the motor through the supply of pressure medium from a pressure medium source, said moment tending to pivot the pivoting cradle in one direction. However, it is also possible for an additional counter pivoting device to be present.
The disadvantage with an axial piston machine of this kind is that, when operated as a pump, it is necessary to reverse the direction of rotation of the drive shaft to reverse the direction of delivery and, when operated as a motor, it is necessary to change the high pressure and low pressure working ports to reverse the direction of rotation of the drive shaft. For the latter case, one known practice in the prior art is to provide a directional control valve which can connect a pressure medium source, e.g. a pump, alternately to one or the other working ports of the motor. The disadvantage of axial piston motors of this kind is the expenditure on equipment involved, resulting from the directional control valve itself and from the two high pressure-proof ports and working lines via which the directional control valve has to be connected to the two working ports.